Although synthetic resin paintbrush bristles have been long known and have even acquired a substantial market in the manufacture of paintbrushes, many paintbrushes continue to be made from hogs' hair. However, hogs' hair presents many difficulties, including expense, fluctuating market supply, and handling difficulties by the paintbrush manufacturer. Advantages of hogs' hair are its naturally flagged tips and its scaly surface which helps hold wet paint in the paintbrush during use. A further disadvantage, in addition to those of high cost and inconsistent availability as pointed out above, is its irregular cross-section as shown in FIG. 2 which is a photomicrograph of a transverse cross-section of a group of hogs' hair bristles, and which clearly illustrates the noncircular cross-section. The circular cross-section is desirable to the paintbrush manufacturer during manufacturing of the brush.
It has long been desired to provide a synthetic paintbrush bristle which has all the attributes of hogs' hair and none of its disadvantages, and the patent literature contains many examples including Ward et al U.S. Pat. No. 4,307,478 and U.S. Pat. No. 4,409,372; Shaw U.S. Pat. No. 3,186,018; Lewis et al U.S. Pat. No. 3,256,545; Grobert U.S. Pat. No. 3,344,457; Curtin et al U.S. Pat. No. 3,706,111; Nakashima U.S. Pat. No. 4,559,268; Alfthen U.S. Pat. No. 2,418,492; Crampton U.S. Pat. No. 3,173,163; and Ingraito U.S. Pat. No. 2,697,009. From these prior patents, it will be seen that it has been proposed to make paintbrush bristles of a variety of cross-sectional shapes, level or tapered, hollow or solid, and formed of a variety of synthetic plastic materials.
Hollow synthetic paintbrush bristles have become very popular and among these the tapered hollow bristle as shown in the aforementioned Ward U.S. Pat. No. '478 has achieved wide acceptance. Also see Ward et al U.S. Pat. No. 4,376,746. One problem, however, is that hollow bristles cannot be satisfactorily tipped, something which is desirable for paintbrush bristles. In addition, the surface of synthetic paintbrush bristles, whether hollow or solid, and whether tapered or level, tends to be relatively smooth, and these synthetic bristles therefore lack the wet paint holding properties of rough surfaced hogs' hair bristles.
The manufacture of cellular or porous bristles or filaments is also known, although no microcellular paintbrush bristle has ever previously been successfully made, insofar as is known. In this regard, paintbrush bristles require a number of particular physical characteristics not necessary in, or contrary to, those of textile yarns and other types of brush bristles, e.g. a high stiffness to weight ratio. Patents which mention porous or cellular broom bristles or porous textile yarns useful for the manufacture of fabrics include Lewis U.S. Pat. No. 3,411,979; Charvat U.S. Pat. No. 3,577,839, Narota U.S. Pat. No. 4,552,810; Halbig U.S. Pat. No. 2,907,096; Fujimura U.S. Pat. No. 4,485,141; Hickman U.S. Pat. No. 3,785,919; Mixon U.S. Pat. No. 3,893,957; Skochdopole U.S. Pat. No. 3,723,240; Okie U.S. Pat. No. 4,144,371; and Bloch U.S. Pat. No. 2,200,946. Textile filaments are very fine and limp, while broom bristles are coarse and very stiff. However, microcellular bristles especially adapted for paintbrush use and having the desired characteristics have either not been previously contemplated or have not been able to be successfully made.
Monofilaments have also been prepared from synthetic materials so as to provide a surface which is rough and irregular. In this regard there may be mentioned Hansen U.S. Pat. No. 3,671,381; Charvat U.S. Pat. No. 3,134,122; Sawkiw U.S. Pat. No. 3,325,845; Ono U.S. Pat. No. 3,567,569; Yamaguchi U.S. Pat. No. 4,254,182; Matsumoto U.S. Pat. No. 4,297,414; and Brody U.S. Pat. No. 4,522,884.
For whatever the reason, a microporous paintbrush bristle having desirable characteristics including the desirable characteristics of the natural hogs' hair bristle, and further having addition advantages over natural hogs' hair bristle, has not previously been available. These characteristics include high "snap-back" stiffness, a high stiffness to weight ratio, cross-sectional uniformity, a rough paint-holding surface, flagged or tipped end, and low price. It is believed that while it might have been known that a cellular paintbrush bristle would be desirable, no one knew how to make such a bristle having the desired characteristics.